Ask 10 random people on the street what nutrients to watch for heart health, and you'll get 10 variations on "saturated fat and cholesterol."
In fact, there's no real evidence that saturated fat and/or cholesterol from whole foods cause heart disease. But did you know about the connection between (some) carbohydrates and heart disease? It's not as simple as "carbs cause heart disease," but there is a connection for certain types of carbohydrates, especially highly refined carbs. Basically, the problem is that sugar and refined carbs the lining of your blood vessels, technically called the vascular endothelium.
Meet your Endothelium
The endothelium is a layer of cells on the inner surface of your blood vessels. A healthy endothelium prevents blood clotting, keeps inflammation under control, regulates the formation of new blood vessels, and controls blood pressure by constricting or dilating your blood vessels. Basically, if you want a healthy cardiovascular system, you need a healthy endothelium.
Poor endothelial function is one of the first signs of heart disease. It can cause high blood pressure (aka hypertension) by constricting blood vessels too much. If the anti-clotting function of the endothelium fails, then you'll be more prone to developing clots.
Blood Sugar and Endothelial Function
This review goes over the relationship between carb quantity, carb quality, and endothelial function.
High blood sugar (hyperglycemia) changes the behavior of the endothelium and damages the endothelial lining of the blood vessels. This is called endothelial dysfunction. In endothelial dysfunction, the endothelium doesn't dilate easily, even when it's supposed to. When the endothelium stays constricted all the time, it causes high blood pressure because the same volume of blood is being forced through the constricted blood vessel.
The technical term you need to know for this is flow-mediated dilation (abbreviated FMD). If someone has normal FMD, then they don't have any endothelial dysfunction and their blood vessels can dilate as appropriate to prevent high blood pressure. If someone has reduced FMD, then they're in trouble - endothelial dysfunction, high blood pressure, the whole shebang.
High blood sugar also increases inflammation in the endothelium. If you really want to get technical and nerdy about it, high blood sugar increases the formation of advanced glycation end products (AGEs) in the blood vessels and also impairs anti-inflammatory action in the endothelium.
But bear in mind that endothelial dysfunction is a problem caused by high blood sugar, not necessarily just eating carbs. Different amounts and types of carbs affect different people's blood sugar in different ways. Some people can eat quite a high-carb diet and have 0 blood sugar problems. Other people start having blood sugar swings if they look sideways at a potato.
Luckily, there have been quite a few studies on specifically what kind of carbs are linked to endothelial dysfunction.
Carbs, Blood Sugar, and Endothelial Function
According to the review, researchers actually first noticed the carb-endothelium connection during oral glucose tolerance tests (OGTTs). Glucose is a type of simple sugar, the fastest type of carbohydrate to digest. In an OGTT, the doctor gives you a pure glucose drink and measures how well your body can cope with the glucose load. The whole point is to make your blood sugar spike and then see how fast your body can bring blood sugar levels back down.
Researchers started noticing that OGTTs caused relatively significant endothelial damage. That was the first sign that blood sugar spikes could cause endothelial problems. On the other hand, they could prevent problems by giving patients vitamin C (an antioxidant) or a statin (statins are anti-inflammatory). This makes sense, since endothelial damage is an inflammatory type of oxidative stress.
An OGTT is a really artificial situation that doesn't really imitate the effect of a normal meal on blood sugar. But on the other hand, an OGTT is basically a super-concentrated sugar dump, and it's not that much different from, say, eating a huge amount of candy or drinking a big glass of Coke. And lots of people eat a lot of highly refined sugar in the form of candy and Coke. So the review went on to examine carb quantity and carb quality as they affect endothelial function.
- Carb quantity is very simple: how many grams of carbohydrate in a given meal or day of eating?
- Carb quality can be measured in all kinds of ways, but for the purposes of this study they were looking at glycemic index (GI). This takes little bit of explanation. The glycemic index is basically a measure of how high a food spikes blood sugar when eaten in isolation. As we've explained before, it has a lot of problems. If you're talking about one food at at a time, it makes no sense to divide "good carbs" from "bad carbs" based only on the glycemic index. But if you're looking at overall dietary patterns, diets with a high GI tend to be high in junk food and refined carbs, so it's pretty reasonable measurement of how much Wonderbread and Coke someone is eating.
When the researchers looked at carb quantity, they actually found that low-carbohydrate diets were associated with poorer vascular health and more endothelial damage, but noted that the effects of increasing fat and protein to compensate naturally obscured any relationship between carb quantity and endothelial health. So they moved on to carb quality.
Carb quality was easier to study. In a couple different human intervention studies, low-GI (read: low-junk) diet were pretty effective for improving endothelial health. This is backed up by various different association studies, which the authors also cited (the review is free full-text, if you want to read it and see all the gritty details). Carb quantity may or may not be important, but the ability of those carbs to cause blood sugar issues (which will obviously vary from person to person) definitely seems to be.
The Long Term: Insulin Resistance and Endothelial Health
Insulin resistance is a measure of long-term blood sugar problems. So looking at people with insulin resistance can give you an idea of how blood sugar problems affect endothelial health in the long term.
This study looked at subjects who started out as either insulin sensitive or insulin resistant. In people with insulin resistance, their endothelial health (measured by flow-mediated dilation) was worse all the time, when they were just walking around. Then it got even worse after a meal, especially a high-carb meal. Unfortunately, the researchers in this study didn't test carb quality at all, only the total number of carbs in the meal.
Also, remember the OGTT results from above? People with diabetes got even worse endothelial problems from an OGTT than healthy people. That's another sign that baseline insulin issues exacerbate the damage of blood sugar spikes.
Insulin resistance, which is a sign of long-term issues with blood sugar regulation, is pretty clearly linked to chronic endothelial damage.
Summing it Up: Carb Quality Counts for Heart Health
Even healthy people will have a transient decrease in endothelial function after a sugar overload like an oral glucose tolerance test. But the real problem here is the long term. In the long term, diets full of refined carbs (high GI) tend to cause chronic endothelial dysfunction. Unsurprisingly, people with insulin resistance have higher levels of endothelial dysfunction. Blood sugar spikes and other blood sugar regulation problems damage the endothelial lining and if that keeps going on, it causes long-term problems.
Endothelial dysfunction is the very first step towards heart disease later on. It makes people more vulnerable to blood clots and raises blood pressure.
Carb quality and blood sugar management matters for heart health. So if you want to keep your ticker going strong, maybe forget about the saturated fat and cholesterol and start focusing on sugar!